An active matrix substrate used in, for example, liquid crystal display devices includes a switching element, such as a thin film transistor (hereinafter, “TFT”) disposed per pixel. As such a switching element, a TFT that has an amorphous silicon film as an active layer (hereinafter, “amorphous silicon TFT”) or a TFT that has a polycrystalline silicon film as an active layer (hereinafter, “polycrystalline silicon TFT”) has been broadly used.
These days, it has been proposed that as a replacement for amorphous silicon or polycrystalline silicon, an oxide semiconductor be used as a material of an active layer of a TFT. Such a TFT is referred to as an “oxide semiconductor TFT”. An oxide semiconductor has higher mobility than amorphous silicon. The mobility of an oxide semiconductor TFT is, for example, 20 times higher than that of an amorphous silicon TFT. Thus, it is possible for an oxide semiconductor TFT to operate more rapidly than an amorphous silicon TFT.
FIG. 12 is a graph illustrating drain current (Id)-gate voltage (Vg) characteristics of an amorphous silicon TFT, a crystalline silicon TFT, and an oxide semiconductor TFT. As shown in FIG. 12, the Id-Vg characteristics of an oxide semiconductor TFT, which has high mobility, rise sharply, which means that off-leakage current is small. The off-leakage current of an oxide semiconductor TFT is less than, for example, 1/1000 of the off-leakage current of an amorphous silicon TFT and less than, for example, 1/10000 of the off-leakage current of a crystalline silicon TFT. Accordingly, an oxide semiconductor TFT has excellent off-state characteristics as well as on-state characteristics.
On the other hand, techniques of disposing a driving circuit, such as a gate driver or a source driver, monolithically (in an integrated manner) on a substrate are known. Such a driving circuit (monolithic driver) typically includes a TFT. These days, a technique of producing the monolithic driver on a substrate by using an oxide semiconductor TFT is used. This achieves cost reduction by a simplified mounting process or a decrease in a picture-frame region. Typically, a TFT constituting a driving circuit (hereinafter, “circuit TFT”) is simultaneously produced in a process of producing a TFT disposed per pixel (hereinafter, “pixel TFT”) as a switching element. Thus, a circuit TFT and a pixel TFT are often formed by using the same oxide semiconductor film and have the same or a similar structure.
An active matrix substrate including an oxide semiconductor TFT is disclosed in, for example, PTL 1. In PTL 1, a plurality of types of oxide semiconductor TFTs having threshold voltages different from each other are formed on a single substrate by varying the elemental ratio of oxide semiconductor layers. The active matrix substrate disclosed in PTL 1 includes a plurality of types of oxide semiconductor TFTs as pixel TFTs, and these oxide semiconductor TFTs are disposed such that the threshold voltage of the pixel TFTs decreases with increasing distance from a gate-line driving circuit.